The number of stop-and-frisks conducted by cops plummeted by an astonishing 34 percent between the first and second quarters of the year, The Post has learned.

A city official with access to the data said 133,934 people were stopped by police between April 1 and June 30, compared to 203,500 from January 1 to March 31.

If the lower pace continues for the rest of the year, the total number of stops will be lower than in 2011. That has happened only once before during Mayor Bloomberg’s entire tenure.

The remarkable drop came after Police Commissioner Ray Kelly, under fire from critics who charged that too many innocent people were being questioned, pledged in May to institute additional oversight, training and outreach to prevent racial profiling.

“That’s part of the reason it went down, but not the biggest reason,” explained the source.

He said rookie cops assigned to high-crime “impact” zones conduct most of the stop-and-frisks and when crime spiked in the beginning of the year the NYPD kept two separate classes of about 1,000 recent academy graduates assigned to that duty.

“Basically, the Police Department doubled down,” the official said.

By April, the rookie complement was back to normal — and the stops dropped sharply.

There are currently 27 neighborhoods categorized as “impact” zones.

There were 1,769 weapons seized in the most recent three-month period, including 194 firearms, 1,259 knives and 316 devices classified as “other.”

The official said that tracked closely to the races of criminals identified by witnesses: 66 percent black; 26 percent Hispanic; 6 percent white; and 2 percent Asian.

The Police Department recorded a record number of 685,724 stops last year.

When the first-quarter number came in at 203,500, critics howled that the city was on pace to reach 800,000 this year and demanded more accountability.

Kelly and Bloomberg have defended the program, even as they announced the new training.

“Anytime you refocus the training, you anticipate the numbers coming down,” said the source.

Among other things, police brass discovered that some cops were unnecessarily classifying legitimate arrests as stop-and-frisks by filling out “250’s,” the forms used to document each stop.

“If a cop observes someone doing something suspicious and has probable cause to go over and makes an arrest, he doesn’t have to fill out a 250,” said the source.

The mayor has responded to critics of his policies by pointing to the city’s murder rate, which is down about 16 percent and is on pace to end the year at the lowest level in recent history. The overall crime rate, however, is up by about 4 percent this year.